Hi. I am hoping someone can help. For the past few months, my 5.4L sounds like a train, when between 1400 and 1900 RPM's. Kind of a whoosh-whoosh-whoosh which seems to be coming from the exhaust. I have changed plugs, and all of the old ones looked good - no dead holes from what I can tell. When this noise happens, the engine power seems to drop off slightly, but not much. Kicking up the RPM's - the noise goes away.

I am stumped. I have visually checked hoses for vacuum leaks but haven't come up with anything. Would a clogged PCV valve do this? Tranny slipping? All ideas welcome.

Thank you!

Bernie